Guitar Vibrato

This simple vibrato adds a tremolo effect to any electronically amplified musical instrument. In other terms the vibrato or tremolo makes the sound to fluctuate at a very low frequency. These fluctuations adds a new dimension of the music.

These device have their greatest uses with Guitars and accordions. The circuit consists of three basic parts. Firstly a very low frequency amplifier; secondly a special type of amplifier using a field effect transistor and thirdly a regulated power supply.

The oscillator consists of a single stage transistorized amplifier where the output circuit is connected to its input through a resistor-capacitor network that produces a phase shift of 1800. Thus the amplified output is fed back to the input in exactly the same phase that is required to produce sustained oscillations. The ladder like three phase shifting network shown in the diagram can produce the phase shift of 1800 at one particular frequency. So the frequency of oscillations in this type of oscillators depends on the values of resistors and capacitors used in phase shifting network. The oscillator circuit is called phase shift oscillator.

The frequency of oscillation can be increased by decreasing the values of resistors and capacitors. The approximate frequency of oscillations is equal to: 1/ 2π √ 6 R C Hertz [Where C = 1 μF, R = 10 K Ω]. The circuit approximately works out of 7 Hertz.

The second stage of the device consists of a Field effect transistor (FET) amplifier. The basic FET consists of a slab made of special silicon semiconductor material. Two electrodes are at two ends and rest two electrodes are at two sides. Two end electrodes are called as “Source” (S) and “Drain” (D). Rest two side electrodes are called as “Gate” (G) and “Shield”. The GATE electrode can be compared with the BASE of a conventional transistor. The SOURCE electrode can be compared with the EMITTER of a conventional transistor and the DRAIN can be compared with the COLLECTOR of a conventional transistor. The electrical path between Source and Drain acts as a simple resistor. The value of the resistor can be increased almost to infinity by applying reverse bias voltage with respect to Source) to the Gate electrode.

The output of the phase shift oscillator is fed to the Source electrode of the FET which varies the amplification of the amplifier according to the frequency of oscillation. The 100K variable resistor (R6) in the circuit controls the magnitude of oscillations fed to the Source electrode of the FET. This 100K potentiometer can adjusted to get proper vibrato or tremolo effect. The switch in the same path is for total removal of the vibrato effect when not required.

The third stage is the regulated power supply. To operate the vibrato/tremolo circuit a stable 18V DC supply with maximum 100ma current source is required and this is very important. One separate power supply has been provided with the circuit. But when the vibrato/tremolo circuit is being used with the main high power amplifier, a stable 18V DC source with the required current may be taken out from the amplifier power supply unit.

On completion of the assembly of the vibrato/tremolo circuit place a DC voltmeter of 0 to 10V range and connect the voltmeter RED probe to DRAIN Terminal and BLACK probe to SOURCE terminal and adjust the 10K preset (R3) so that, the voltmeter reads 2 volts Now the device is ready for use.